To investigate the molecular structure and micropore structure of organic matters in coal-measure shale, the black shale samples of the Shanxi formation were collected from Xishan Coalfield, Taiyuan, and a hybrid experimental–simulation method was used for realistic macromolecular models of organic matter (OM). Four experimental techniques were used to determine the structural information of OM, including elemental analysis, state ¹³C nuclear magnetic resonance (¹³CNMR), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FTIR). With structural parameters, two-dimensional (2D) average molecular models of OM were established as C₁₇₇H₁₆₀O₈N₂S with a molar weight of 2474, which agreed well with the experimental ¹³C-NMR spectra. A realistic three-dimensional (3D) OM macromolecular model was also reconstructed, containing 20 2D molecules with a density of 1.41 g/cm³. To determine the connectivity and spatial disposition of the OM pores, focused ion beam microscope (FIB-SEM) and transmission electron micrographs (TEM) were utilized. The 3D OM pores models were developed. The results show that whether the OM pores varied from 20 to 350 nm as obtained from FIB-SEM images or less than 10 nm as observed in the TEM images, both were of poor connectivity. However, the ultra-micro pores from the 3D OM macromolecular model varied from 3Å to 10 Å and showed certain connectivity, which may be the main channel of diffusion. Furthermore, with the pressure increased, the methane adsorption capacity of the 3D OM model increased with a maximum value of 103 cm³/g at 7 MPa, indicating that OM pores less than 1 nm have a huge methane adsorption capacity. Therefore, our work provides an analysis method that is a powerful and superior tool in further research on gas migration.

As the essential component of shale, clay minerals have a vital influence on the pore structure and the gas content of reservoirs. To investigate the compositional characteristics of coal-measure shale and its effects on pore structure, a total of thirteen Taiyuan formation shale samples were collected from the Qinshui Basin and were analyzed using a combination of X-ray diffraction analysis, X-ray fluorescence spectrometry, Fourier transform infrared spectroscopy (FE-SEM), polarized optical microscopy, and field emission scanning electron microscopy. The results show that the principal minerals of the samples are quartz, kaolinite, and illite. Most of the kaolinite was an original terrigenous detrital material with low crystallinity and a low degree of ordering, whereas the illite was mainly composed of 1M_d resulting from diagenesis. Clay minerals developed slits, irregularly-shaped or multisized pores during diagenesis, which can be classed into interlayered pores, intergranular pores, and microfractures. Eight micro-morphological forms of clay minerals were summarized based on FE-SEM observations, such as compacted, parallel, bent, tilted, mutually supporting structures, etc., which are mainly formed by the mechanical compaction of clay minerals with different sizes, shapes, and contact relationships. The diversity and complexity of the micro-morphological forms of clay minerals contribute to the strong heterogeneity, low porosity and high permeability anisotropy of shale.

Hydraulic fracturing is a widely used stimulation technology in coalbed methane (CBM) fields. However, the coal reservoir damage caused by high-pressure hydraulic fracturing seriously affects the production effects, and the mechanism is not clear. Therefore, based on high-pressure water injection (HPWI), Fourier transform infrared spectroscopy (FTIR), and contact angle tests, the effects of HPWI on surface chemical properties and wettability of different rank coals were studied. The FTIR results show that surface functional groups of different rank coals have changed to varying degrees after HPWI. After HPWI, the content of Ash in Shaqu and Yonghong coal decreases by 2.29% and 27.91%, while it increases by 297.87% in Shaping coal. The C–O bond content in Shaping and Yonghong coal decreases by 6.32% and 15.19%, while the C–O bond content in Shaqu coal increases by 50.96%. The content of C=O in Shaping and Yonghong coal increases by 2.44% and 27.84%, respectively. The R₂CH₂ contents increase by 19.75% and 12.5% in Shaping and Shaqu coal, while decreasing by 6.48% in Yonghong coal. The RCH₃ content increases by 21.11% in Yonghong coal, while it decreases by 19.09% and 24.01% in Shaping and Shaqu coal. The content of cyclic associated hydroxy–hydrogen bond decreases by 41.25%, 63.92% and 65.86% in Shaping, Shaqu, and Yonghong coals, and the content of free hydroxyl group increases by 57.92%, 58.42%, and 93.71%. The f_ar^c of coal remains almost unchanged, the DOC increases by 20.21%, 126.77% and 0.24% in Shaping, Shaqu, and Yonghong coals, and the I decreases by 16.67% and 51.46% in Shaping and Yonghong coals, indicating that the ordering of coal becomes better, and the content of methylene carbon in the form of long straight chain increases after HPWI. The complexity and differences of changes in functional groups are mainly due to differences in coal structures caused by coalification. The contact angle tests show that the wetting contact angle of different rank coals decreased by 2.30% to 14.50%, revealing that the hydrophilicity of coals increases after HPWI. The decline rate of wetting angles in medium and high-rank coals was significantly higher than that of low-rank coal. This phenomenon discovered that the increase in hydrophilic functional groups caused by HPWI action leads to an increase in the hydrophilicity of coal samples, which is not conducive to the drainage efficiency in CBM development.